The present invention relates generally to the field of oxidant heaters (air heaters and air pre-heaters) for use in coal-fire power plants and in particular to a system and method for use in oxy-fuel combustion which incorporates a novel regenerative oxidant heater internal sector arrangement as well as the strategic positioning of the primary oxidant fan and primary oxidant mixer.
Oxygen combustion (“oxy-fuel combustion”) is a means of drastically decreasing the amount of nitrogen in the flue gas from a boiler firing a carbonaceous fuel in order to achieve a much higher concentration of carbon dioxide (CO2) in the combustion gasses to permit compression and storage. An oxidant, such as pure oxygen, and a carbonaceous fuel, such as coal, is introduced into the boiler furnace where the fuel is ignited and burned. The resulting gaseous combustion product will contain primarily CO2 along with some water and various compounds and oxides depending on the fuel composition. This gas is then further purified and compressed as needed to suit pipeline and storage requirements.
The oxy-fuel combustion process offers several configurations, each having its advantages and disadvantages. In one configuration, the hot flue gas flowing from the oxidant heater outlet is split into primary and secondary streams. Oxygen is then mixed with these streams and they are recycled back to the boiler as primary and secondary oxidant to provide dilution of the flame temperature and maintain gaseous volume for convective heat transfer.
Oxidant may be introduced into the boiler system in several locations and the mixture of oxidant and flue gas is generally heated before it enters the combustion process. Prior to entering the combustion process the recycled gas streams undergo various flue gas treatment processes, which may include removal of particulate matter, SO2 scrubbing, and moisture reduction processes.
In one process variation, warm recycle, the flue gas leaving the oxidant heater is immediately split into two streams. One stream passes through particulate, SO2, and moisture removal as described herein, before being further split between the primary and exit stream to a CPU. The other, secondary stream, passes through particulate removal, and is routed back to the oxidant heater in a “warm” state (about 400 F).
In conventional combustion systems, the most widely used and lowest cost type of oxidant heaters which are employed to heat the combustion oxidant (i.e., air) are regenerative oxidant heaters (“air heaters” or “air pre-heaters”).
Regenerative oxidant heaters transfer heat indirectly by convection as a heat storage medium is periodically rotated into the hotter and cooler flow streams. In steam generating plants, tightly packed bundles of corrugated steel plates serve as the storage medium. In these units either the steel plates, or surface elements, rotate through oxidant (or air) and gas streams, or rotating ducts direct oxidant and gas streams through stationary surface elements.
The conventional regenerative oxidant heater which is most commonly used is the Ljungstrom® type which features a cylindrical shell plus a rotor which is packed with bundles of heating surface elements which are rotated through counterflowing oxidant and gas streams. FIG. 1 shows the typical positioning of the sectors through which the primary oxidant stream 51 and secondary oxidant stream 52 as well as the counterflowing flue gas stream 50 flows in a conventional Ljungstrom® type regenerative oxidant heater. As is shown, oxidant flows through one half of the rotor and flue gas which comes from the boiler's gas exit flows through the other half. Additionally, the oxidant side (or “air side”) consists of two sectors, one for the primary stream and one for the secondary stream.
Another common conventional regenerative oxidant heater sector arrangement includes that which is disclosed in U.S. patent application 2006/0090468 filed by Counterman (“Counterman”). The regenerative oxidant heater of Counterman is adapted to receive a flow of cool oxidant in counterflow to the flow of hot flue gas and to provide a heat exchange between the cool oxidant and the hot flue gas to convert the cool oxidant into the heated combustion oxidant. FIG. 2 (Prior Art) is a bottom cross sectional view of the rotor of an oxidant heater with a typical sector arrangement, which is also employed in Counterman. It shows the primary oxidant sector 61, and the secondary oxidant sector 62, through which respective primary and secondary oxidant streams flow toward a boiler. The primary and secondary sectors 61, 62 are separated by a sector plate 63 and they are both adjacent to the flue gas sector 60, through which flue gas flows away from a boiler. Additionally, the primary and secondary sectors 61, 62 are separated from the flue gas sector 60 by sector plates 64 and 65.
In conventional regenerative oxidant heaters for an air-fired power plant, a typical pressure for the primary oxidant stream (or “air stream”) is high as compared with that of both the secondary air stream and the flue gas stream. For example, a typical pressure for the primary oxidant stream is about +40 inches of water gage (in. wg), for the secondary oxidant stream it is about +20 in. wg, and for the flue gas stream from the boiler it is about −5 in. wg. Thus, between the secondary air sector and gas side there is about a +25 inches water gauge difference in air pressure which can result in 14% leakage of the oxidant into the gas side. Furthermore, the pressure gradient between the primary air sector and the gas side is much greater.
In all regenerative heaters which use conventional sector arrangements such as those discussed above, the rotating heat exchanger in combination with the pressure differential between the respective streams causes inherent leakage between the air (or oxidant) side and the gas side conveying the hot flue gas from the boiler.
Thus, a further known oxidant heater sector arrangement as shown in FIG. 3 splits the secondary oxidant stream 72 in two and positions it in two secondary air sectors adjacent to the gas side 70. Here, the primary sector 71 through which the primary oxidant stream flows is positioned between the two secondary sectors in order to minimize the pressure difference to the greatest extent possible in an attempt to reduce leakage and optimize performance.
However, in oxy-fuel combustion, all these configurations result in leakage from the oxidant to the gas side. In addition most of the oxygen must be added to the recycled flue gas upstream of the oxygen heater in order to achieve a reasonable flue gas temperature leaving the oxidant heater. Thus any leakage results in the loss of costly oxygen along with recycle gas to the exit stream thereby necessitating additional oxygen production in an air separation unit (ASU) to make up for the loss as well as removal of additional oxygen from the product gas in a compression and purification unit (CPU) before the concentrated CO2 can be disposed of. A need exists to minimize such leakage as the cost of additional oxygen production and removal is prohibitive.
An additional problem encountered when using an oxidant heater employing a conventional sector arrangement in oxy-fuel combustion, is difficulty achieving acceptable exit gas temperature. Due to the high temperatures of the flue gas and oxidant streams, it can be very difficult, especially evident in warm recycle, to achieve an acceptable gas temperature at the oxidant heater exit.
Furthermore, the concern about costly oxygen loss as well as the problem caused by the high temperature of the oxidant(s) and flue gas makes it difficult to achieve a design that balances both acceptable heat exchanges within the oxidant heater as well as reasonable oxidant heater exit gas temperatures.
When attempting to achieve such a balance, a typical known step is to add cool oxidant to the oxidant flow stream(s) prior to (i.e., upstream from) the oxidant heater. However, this conventional method is undesirable because leakage from the oxygenated oxidant stream(s) into the boiler flue gas stream results in the loss of costly oxygen. The loss of oxygen occurs because a substantial portion of it will flow in the stream which leads to the compression process. Additionally, if the oxygen is added to the primary or secondary oxidant stream after the oxidant heater an acceptable oxidant heater design becomes more difficult to achieve.
Another common approach which has been taken in an attempt to reduce leakage and hence loss of costly oxygen, is to locate the primary oxidant fan downstream of the oxidant heater. However, this approach has consistently been ineffective.
Additionally, a known alternative to using a regenerative oxidant heater in order to eliminate internal leakage and avoid loss of costly oxygen, is to use expensive separate primary and secondary tubular or plate type pre-heaters for the secondary and primary oxidant streams which completely separates the oxidant side and the gas side allowing no leakage. However, although this alternative may be reasonable for use on the industrial boiler scale in air fired applications, it is not cost effective when applied to large utility boilers. Additionally, separate tubular pre-heaters require considerably more space than regenerative oxidant heaters and tubular pre-heaters are susceptible to significant internal leakage with age, thus inevitably resulting in oxidant loss.
Accordingly there is a clear need for a cost effective system and method which incorporates a regenerative oxidant heater design that will minimizes the loss of costly oxygen normally caused by internal oxidant heater leakage and which will also achieve an acceptable balance between reasonable heat exchange within the oxidant heater and a reasonable oxidant heater exit flue gas temperature.